The present invention involves irradiation of gas mixtures such as combustion gases and particularly flue gases, to facilitate removal of certain contaminants. More specifically, a gas stream is optically "pumped" to enhance reactivity and to convert sulfur and nitrogen oxide contaminants to oxidized and more readily nucleated species for recovery of those contaminants in an electrostatic precipitator.
It is generally known that certain chemical and physical processes in systems of interacting molecular species can be induced to occur at accelerated or decelerated rates by selective excitation of certain species. Such excitation by electromagnetic radiation in or near the visible spectrum is sometimes referred to as optical pumping. Optical pumping may increase the population of the particular molecular states where the interaction matrix elements have intrinsically large values, or molecular states where the interaction threshold energy requirements can be met at a given temperature of the reaction system. A well known example of the first case is the formation of photochemical smog and of the latter is optically pumped lasers.
The present invention is concerned with optical excitation of gas mixtures containing sulfur and nitrogen oxides in order to promote oxidation and nucleation of those materials. More highly oxidized forms of sulfur and nitrogen oxides may then be scrubbed from effluent gas streams by conventional techniques, such as electrostatic precipitation.
Typical of the prior art regarding photochemical reactions of sulfur dioxide and oxides of nitrogen is U.S. Pat. No. 4,097,349 to Zenty. As taught therein, gas streams containing hydrocarbons, oxygen, sulfur dioxide and oxides of nitrogen (NO.sub.x) can be subjected to ultraviolet light in order to promote a variety of reactions. As disclosed by Zenty, light having a wavelength of from 240 nanometers to 340 nanometers is typical of that absorbed by sulfur dioxide (column 3, lines 12-15). Equation 17 of the Zenty patent demonstrates the absorption of 290-340 nanometer ultraviolet radiation by sulfur dioxide to produce a singlet SO.sub.2. Equation 18 of the Zenty patent demonstrates the absorption of 340-400 nanometer ultraviolet radiation by SO.sub.2 to produce triplet SO.sub.2. Zenty further discusses the transformation of singlet SO.sub.2 to triplet SO.sub.2 which is then reacted with another species present in the gas stream to quench the triplet (excited state) sulfur dioxide. In all cases, the production of excited state sulfur oxides precedes the quenching of the excited state species by materials including nitrogen, oxygen, water, carbon monoxide, carbon dioxide, nitric oxide, ozone, methane, and other hydrocarbon species. It must be noted, however, that Zenty does not suggest or teach a reaction of ground state (unexcited) oxides of sulfur in order to remove such species from a gas stream. Although the Abstract and claim 1 of this patent indicate irradiation with a wavelength of 150 to 750 nanometers, the specification does not indicate what effects, if any, are produced with radiation having a wavelength below 240 nanometers.
An alternative process for the reduction of sulfur and nitrogen oxide contaminants in effluent gas streams is taught by Richards in U.S. Pat. No. 3,984,296. This reference teaches the formation of electron donor-acceptor molecular complexes (EDA complexes) in flue gas by the exposure of the flue gas to Lewis acids or bases formed electrostatically within a corona precipitator. Alternatively, Richards suggests a photochemical technique for the production of the EDA complexes using infrared radiation of 400 to 1,000 nanometer wavelengths or ultraviolet radiation of 120 to 240 nanometer wavelengths.
The Richards patent further teaches a photo-induced oxidation of the EDA and reaction of sulfur dioxide (or NO.sub.x) with EDA contained in stack gas. For this part of the process, the patent teaches the use of ultraviolet light having a wavelength of 150 to 500 nanometers and optimally between 300 and 400 nanometers in order to carry out the photo-oxidation of the EDA complex. Further, Richards suggests that free radical reactions may occur under the influence of ultraviolet radiation (column 8, lines 12-15). Apart from these broad and non-specific teachings, an understanding of the reactions disclosed by Richards in the Table at column 9, lines 1-21, of the patent reveals that the underlying reaction mechanisms required for practicing the Richards patent are identical to those disclosed by Zenty as reactions 17, 18 and 19 of U S. Pat. No. 4,097,349. This is particularly apparent because of the preferred wavelength taught by Richards (300 to 400 nanometers) and the lack of any suggestion or teaching by Richards of the formation of ground state atomic oxygen.
Finally, Machi et al (U.S. Pat. No. 3,869,362) disclose a process for removing sulfur and nitrogen oxides from effluent gas streams. As claimed, the Machi process includes irradiation of effluent gases with "ionizing radiation or ultraviolet light."
Although generally mentioned at several places in the patent (col. 1, line 5; col. 5, line 47; abstract), no examples of any radiation source except a Cockcroft-Walton type election beam generator are given. In addition, no specific parameters relating to either an ultraviolet-driven reaction or to ultraviolet light ar presented.
The extremely general nature of the teaching of the '362 patent, taken together with the later Machi et al patent (4,004,995) (which did not continue the earlier patent's suggestion of the interchangeability of electron beams and ultraviolet light) suggest that any reactions noted by Machi et al using ultraviolet light were of the type demonstrated by Zenty. This is made clear by the concentration-dependency of the Machi removal method which is not a characteristic phenomenon of the mechanism of the present invention.